1. Field of the Invention
The present invention relates to a developing apparatus and a process cartridge that adopts a non-contact development system using a magnetic mono-component developer for making visible the electrostatic latent image formed on an image bearing member by way of an electrophotographic printing method, an electrostatic recording method, and the like.
2. Description of the Related Art
In recent years, there is a growing need for more compact and higher speed image forming apparatus, in which images are formed by the electrophotographic printing method, the electrostatic recording method, and the like employed as a printer or copier for personal use. In addition, in view of the maintenance of such apparatus, convenience is sought for a developing unit/a cleaning unit including a toner/a waste toner which can be detachably mountable to the main body of the apparatus.
In order to achieve the compactness, the image bearing member, the developer carrying member, and the like used in the apparatus are required to make the diameter thereof smaller. In particular, in a non-contact development system (i.e., toner projection development (jumping development)) as described in Japanese Patent Application Laid-Open No. H06-110324, as the diameter of a photosensitive drum, which serves as an image bearing member, or a developing sleeve, which serves as a developer carrying member, are made smaller, the developing region is also made smaller. A developing sleeve, which has a diameter of 12 mm or less, has been required to achieve compactness.
The aforementioned term “developing region”, as shown in a region “X” in FIG. 6 of the accompanying drawings, denotes a region in which a bias voltage applied between a photosensitive drum 1 and a developing sleeve 41 and an alternating electric field formed by the latent image potential allow the toner to fly and be involved in the developing process. In conjunction with the present invention, the developing region will be described in greater detail hereinbelow.
The electric field mentioned above is set in such a way to prevent the occurrence of an electric discharge at a position nearest to the photosensitive drum 1 and the developing sleeve 41. The intensity of the electric field will become weaker, as shown in FIG. 6, as the photosensitive drum 1 and the developing sleeve 41 move in a transverse direction with reference to the nearest position in FIG. 6, due to the fact that the distance between the photosensitive drum 1 and the developing sleeve 41 is made wider. As a matter of course, as the photosensitive drum 1 and the developing sleeve 41 have a smaller diameter, (i.e., as each of them has a greater curvature) the photosensitive drum 1 and the developing sleeve 41 will rapidly have a greater distance therebetween, leading to a rapidly weaken intensity of the electric field. Accordingly, the range of the intensity of the electric field sufficient for the toner 43 to fly can be limited to the vicinity of the nearest position.
The first harmful effect caused by a narrower developing region is a decline in the density due to an insufficient toner supply. When various developing conditions are changed in order to compensate for the decreased density and maintain an appropriate density level, there may be a case that a fogged image or an uneven density can occur as described in Japanese Patent Application Laid-open No. H06-110324.
In use of a magnetic toner, the magnetic force contained in the developing sleeve should be made weaker as a measure to prevent the aforementioned problems from occurring. In this manner, a magnetic binding force applied to the magnetic toner on the developing sleeve can be weak so that the toner can fly easily while preventing the decline in the density.
This can certainly widen the developing region and prevent the decline in the density, however, the toner that has not been sufficiently charged (low toriboelectricity) also flies to increase the risk of the fogged image or the spatter of the toner in the apparatus.
The magnetization of the magnetic toner induced by the magnetic force of the magnet can be lowered for the toner to fly easily. For this purpose, there is a case shown in Comparative Example 2 in Japanese Patent Application Laid-open No. H06-110324 wherein a magnetic toner with a lower residual magnetization is used, however, more fogged image and uneven density were observed and thus considered to be not appropriate for practical use.
In the toner projection development, the behavior of the magnetic toner with lowered residual magnetization is described in Japanese Patent Application Laid-Open No. 2005-345618. It shows that the magnetic brush of the magnetic toner that is under the magnetic field can be easily broken and can be close to a state of a toner cloud in which each of the magnetic toner particles separately behaves when the residual magnetization of the magnetic toner is low.
It is further suggested in Japanese Patent Application Laid-Open No. 2005-345618 that the magnetic brush of the magnetic toner can be more easily broken as the degree of circularity of the toner particles is higher. It is yet further suggested in Japanese Patent Application Laid-Open No. 2005-345618 that a toner projection development in a state of a cloud can reduce a so-called edge effect in which the magnetic toner is gathered to the edge of the latent image, and bring out an effect in which a difference between the solid image portion and the line image portion is smaller.
Moreover, in accordance with a tendency of a developing sleeve smaller in diameter (12 mm or less in diameter), the number of revolutions of the developing sleeve per page increases and thus the risk of fusion bonding of the toner on the developing sleeve increases. In accordance with the high speed printing using the developing sleeve with a smaller diameter and the greater durability (longer life) of the developing apparatus, the shortage in the toner supply and the toner fusion bonding on the developing sleeve described above tend to deteriorate, and hence there are many restrictions on achievement of this.
The toner projection development with a small developing region must be conducted under various constraints. In particular, in the case where the developing sleeve has a diameter of less than 12 mm, the toner supply shortage can be caused even when the amount of charged toner as described in Japanese Patent Application Laid-Open No. H06-110324 is maintained and the density cannot be maintained at a continuous output of the images with a high coverage rate.
As a method of physically increasing the amount of toner supply, it may be conceived that the ratio of the circumferential speed of the developing sleeve to the photosensitive drum can be raised. However, it is not desirable in that the number of revolutions of the developing sleeve is increased as described above. Also, a method of changing the condition of regulating the amount of the toner on the developing sleeve to increase the toner carrying amount makes it difficult to obtain an appropriate triboelectricity or triboelectricity distribution, leading to a harmful effect caused by the fogged image, the uneven density, and the like and increasing the possibility of degrading the image quality.
It is an effective measure to provide a larger electric field from the developing sleeve to the photosensitive drum by changing the developing bias. However, in many cases, the maximum value of the electric field has already been set nearly to an upper limit, in which no electric discharge occurs in the nearest position of the photosensitive drum and the developing sleeve, and thus the value cannot be made higher any more.